Typical structure of an air conditioning system for a vehicle equipped with a compressor with variable capacity comprises, as represented by FIG. 1 of the present invention, a compressor, a condenser, an expansion valve, an evaporator, a control valve for controlling output of the compressor, and a valve control module for the control valve according to various detected values. The valve control module measures various values such as the outlet port temperature of an evaporator and speed, inside temperature and outside (exterior) temperature of the vehicle, etc., thereby to adjust the displacement of the control valve.
A compressor used in an air conditioning system for a vehicle is a part that compresses a gaseous refrigerant discharged from the evaporator under low pressure and discharges to a condenser the refrigerant which is highly pressurized so as to be easily liquefied. An example of a commonly used compressor is a swash plate type compressor. In such a swash plate type compressor, a piston that compresses the refrigerant in a compressing chamber while moving reciprocally depending on the rotation of a swash plate is loaded therein.
The structure of a typical swash type compressor (C), as illustrated in FIG. 2, comprises: a front and a rear housing (20) (30) which form a closed space inside such as a crank chamber (21), an intake chamber (30a) and a discharge chamber (30b); a cylinder block (70) which comprises a plurality of cylinder bores (not represented) arranged in circumferential direction and is installed between the front housing (20) and the rear housing (30); a driving shaft (40) that passes through the center of the front housing (20) to be inserted to the crank chamber (21) while being rotatably supported in the center of the cylinder block (70); a plate (50) being rotatably connected to the driving shaft (40) depending on the rotation of the driving shaft (40) in the crank chamber (21); a swash plate (60) that is mounted surrounding the driving shaft (40), is rotatably coupled to the plate (50) so that, depending on the rotation of the plate (50), it can be rotated while being slid in the axial direction of the driving shaft (40) to vary its inclination angle; a plurality of pistons (10) which are coupled with a shoe (62) provided along the circumference of the swash plate (60) to conduct reciprocal movement in each bore of the cylinder block (70) depending on the rotation of the swash plate (60); a valve unit (not represented) that is provided between the cylinder block (70) and the rear housing (30); a control valve (90) for adjust the feed amount of the piston (10); and a spring (80) which is elastically mounted between the plate (50) and the swash plate (60) so as to support the swash plate (60) at the minimum inclination angle when the plate (50) is not rotated.
A means for controlling the output of such swash type compressor is a control valve (90) that adjusts the feed amount of the piston (10), and the control valve (90) is also controlled according to the relation between the discharge pressure and the intake pressure of the compressor, or the displacement of the control valve (90) can be controlled according to the duty ratio of the electric current on excitation coils.
As for other means to control the displacement of a control valve (90), Japanese patent laid-open Nos. 2001-227825 and 2001-227826 disclose a method for forming a map table of a target temperature (referred as refrigerant capacity) that is corresponding to an evaporator outlet temperature (or referred as refrigerant capacity) and a duty ratio of the electric current for achieving the target temperature (or referred as refrigerant capacity). In those laid-opens, the control of the output of a compressor with variable capacity is conducted by driving the control valve according to the current duty ratio corresponding to the measured evaporator outlet temperature in the map table.
As another example related to the control of a control valve, Japanese patent laid-open No. 2001-153425 discloses a method for controlling a compressor control valve according to the lower control value determined by comparing a PID controlling method or a map controlling method.
Further, a method for controlling a control valve by measuring the inside temperature in addition to measurement of the evaporator outlet temperature or refrigerant capacity is also well known in this art.
FIG. 3 illustrates, as described above, a variable mechanism of cooling performance of an air conditioning device through control over a control valve.
FIGS. 4a, 4b and 4c represent a block diagram of controlling the duty value (duty (t)) of a control valve according to a feedback value determined by measuring refrigerant discharge capacity (q(t)), discharge temperature (T.sub.discharge(t)) and inside temperature (T.sub.incar(t)).
In a mode as shown in FIG. 4a which controls the duty value (duty(t)) of a control valve with a refrigerant discharge capacity (q(t)) as a feedback value, a numerical analysis on the air conditioning system as well as a number of complicating sensor inputs for measuring the refrigerant capacity are required, and the inside temperature is likely to be unstable.
Further, in a mode as shown in FIG. 4b which controls the duty value (duty(t)) of a control valve with a discharge temperature (T.sub.discharge(t)) as a feedback value, the control mechanism is complicated since the control valve adjustment is involved with the inside temperature control. It means that a lot of independent control variables such as an opening degree of a control valve and an opening degree of a temperature control door and the like should be concerned, thereby making the control difficult.
In a mode as shown in FIG. 4c which controls the duty value (duty(t)) of a control valve with an inside temperature (T.sub.incar(t)) as a feedback value, responding properties are greatly decreased and temperature control becomes unstable due to excessive delay time. In addition to those problems, an operational load in the compressor would be likely to fluctuate excessively.
Specifically, a conventionally used mechanical control valve utilizes the discharge pressure and the intake pressure of a compressor in its control mechanism. In such mechanical control valve, the actual evaporator temperature is used only by a predetermined cut-off value for preventing icing on the evaporator, therefore it is impossible to carry out fine control over air conditioning.
A conventional external input-type control valve that controls the discharge capacity of a compressor according to various vehicle information such as driving information inputted from ECU of the vehicle, can improve the rate of fuel consumption, however it has problems such as a complicated structure owing to use of a circuit or a sensor which is required for taking the inputted information such as acceleration or decrease in speed, engine condition and velocity, etc., and additional cost for building such circuit or sensor.